Monolithic body mems devices

ABSTRACT

A technique decouples a MEMS device from sources of strain by forming a MEMS structure with suspended electrodes that are mechanically anchored in a manner that reduces or eliminates transfer of strain from the substrate into the structure, or transfers strain to electrodes and body so that a transducer is strain-tolerant. The technique includes using an electrically insulating material embedded in a conductive structural material for mechanical coupling and electrical isolation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119 of provisionalapplication No. 61/831,324, entitled “Monolithic Body MEMS Devices,”filed Jun. 5, 2013, which application is hereby incorporated byreference.

BACKGROUND

1. Field of the Invention

The invention is related to microelectromechanical systems (MEMS).

2. Description of the Related Art

In general, microelectromechanical systems (MEMS) are very smallmechanical devices. Typical MEMS devices include sensors and actuators,which may be used in various applications, e.g., resonators (e.g.,oscillators), temperature sensors, pressure sensors, or inertial sensors(e.g., accelerometers or angular rate sensors). The mechanical device istypically capable of some form of mechanical motion and is formed at themicro-scale using fabrication techniques similar to those utilized inthe microelectronic industry, such as using lithography, deposition, andetching processes.

In general, a MEMS transducer converts energy between different forms,e.g., electrostatic and mechanical forms. MEMS transducers may be usedas both sensors that convert motion into electrical energy(accelerometers, pressure sensors, etc.) and actuators that convertelectrical signals to motion (comb drive, micromirror devices,resonators). MEMS devices using capacitive transducers are easy tomanufacture and result in low noise and low power consumption sensorsand/or actuators.

Capacitive sensing is based on detecting a change in capacitance of acapacitor. If a known voltage is applied across the capacitor (e.g.,fixed DC potential differences applied across the capacitors a the MEMSdevice), changes in current due to capacitive variations will appear inresponse to motion of one plate of the capacitor relative to anotherplate of the capacitor. Similarly, capacitive actuation is based onvariation in electrostatic forces between the two plates of a MEMScapacitive transducer. For example, a DC operating point can beestablished by applying a DC bias voltage across the capacitor and an ACvoltage causing changes in force on a plate of the capacitor.Transduction of a MEMS device is based on the voltage across thetransduction gap generating an electrostatic force, or inversely,transduction based on the gap variation due to displacement generating acharge variation at the output of the transducer. The transduction gapmay vary as a function of environmental factors (e.g., temperature,strain, and aging), thereby changing the capacitance with respect totime. These same environmental factors can also affect the springconstant (i.e., spring stiffness) associated with a MEMS device, whichis typically modeled as a mass-spring-damper system. In general, achange in the electrode capacitance affects the equivalent springstiffness through electrostatic pulling, which affects the resonantfrequency of the MEMS device. MEMS devices targeting applicationsrequiring high-precision (e.g., resonators having resonant frequencyspecifications required to be within +/−10 parts-per-million (ppm)) maynot achieve the target specification due to effects of environmentalfactors on the resonant frequency.

A MEMS device may be configured as a resonator that is used in timingdevices. The resonator may have a variety of physical shapes, e.g.,beams and plates. The MEMS device may have a portion suspended from thesubstrate (e.g., a suspended mass, body, or resonator) attached to thesubstrate by an anchor. An exemplary suspended mass may be a featuresuch as, but not limited to, a beam, a plate, a cantilever arm, or atuning fork. In a specific embodiment, a MEMS device includes aresonating feature (e.g., suspended mass) flanked by one or more driveelectrodes and one or more sense electrodes.

Referring to FIG. 1, a conventional MEMS device (e.g., MEMS device 100)includes resonator 105 coupled to substrate 102 via anchor 104. Duringoperation, electrode 110 electrostatically drives resonator 105 todynamically deflect, which increases a capacitance between resonator 105and electrode 110 when a voltage differential exists between resonator105 and electrode 110 by decreasing the gap between resonator 105 andelectrode 110. Since electrode 110 and resonator 105 are the same heightand thickness and are in the same plane, resonator 105, when driven,deforms laterally, i.e., parallel to the plane of the substrate, acrossa distance between electrode 110 and a second electrode 111. Electrode110 is substantially parallel to substrate 102. Electrode 111 detectsthe resonant frequency of resonator 105 as the capacitance variesbetween resonator 105 and electrode 111 in response to the deflectiondriven by electrode 110. MEMS device 100 is commonly referred to as an“in-plane” or “lateral” mode resonator because resonator 105 is drivento resonate in a mode where the resonator 105 moves laterally (indirection 109) and remains aligned vertically with electrode 110.

Referring to FIG. 2, in an exemplary MEMS application, MEMS device 100is coupled to amplifier 210 in an oscillator configuration. Senseelectrode 202 provides a signal based on energy transfer from avibrating resonator of MEMS device 100, thereby converting mechanicalenergy into an electrical signal. In general, bias signals introduced atvarious points of the circuit determine an operating point of thecircuit and may be predetermined, fixed DC voltages or currents added toAC signals. The resonator of MEMS device 100 receives a DC bias voltage,V_(MASS), which is generated by a precision voltage reference or voltageregulator of bias generator 206. However, in other embodiments, biassignals may be introduced at the electrodes and/or other nodes of theoscillator circuit. A large feedback resistor (R_(F)) biases amplifier210 in a linear region of operation, thereby causing amplifier 210 tooperate as a high-gain inverting amplifier. The MEMS oscillator sustainsvibrations of MEMS device 100 by feeding back the output of amplifier210 to a drive electrode of MEMS device 100. Amplifier 210 receives asmall-signal voltage from sense electrode 202 and generates a voltage ondrive electrode 204 that causes the resonator of MEMS device 100 tocontinue to vibrate. MEMS device 100 in combination with capacitances C₁and C₂ form a pi-network band-pass filter that provides 180 degrees ofphase shift and a voltage gain from drive electrode 204 to senseelectrode 202 at approximately the resonant frequency of MEMS device100.

For some MEMS applications (e.g., a low-power clock source), alow-power, high-Q (i.e., quality factor), stable, and accurateoscillator may be required. However, the power, accuracy, and stabilityspecifications may be difficult to achieve using the conventional MEMSdevice of FIG. 1. Accordingly, improved MEMS devices, e.g., MEMS devicesthat reduce or eliminate factors that affect accuracy and reliability ofthe output frequency of the MEMS device, are desired.

SUMMARY

A technique decouples a MEMS device from sources of strain by forming aMEMS structure with suspended electrodes that are mechanically anchoredin a manner that reduces or eliminates transfer of strain from thesubstrate into the structure, or transfers strain to electrodes and bodyso that a transducer is strain-tolerant. The technique includes using anelectrically insulating material embedded in a conductive structuralmaterial for mechanical coupling and electrical isolation.

In at least one embodiment of the invention, an apparatus includes aMEMS device that includes a body suspended from a substrate. The MEMSdevice includes a first electrode suspended from the substrate. Thefirst electrode and the body form a first electrostatic transducer. TheMEMS device includes a second electrode suspended from the substrate.The second electrode and the body form a second electrostatictransducer. The first and second electrodes are mechanically coupled tothe body. The body, the first electrode, and the second electrode may becoplanar structures formed in a structural layer formed using thesubstrate. The MEMS device may include an anchor mechanically couplingthe first electrode, the second electrode, and the body to thesubstrate. The anchor may include first, second, and third anchorportions mechanically coupled to each other and electrically isolatedfrom each other. The anchor may include electrical insulator portionsdefining portions of the first electrode, the second electrode, and thebody. The anchor may include electrical conductor portions formingportions of the first electrode, the second electrode, and the body. Theapparatus may include a suspended passive element mechanically coupledto the body and electrically isolated from the body. The MEMS device mayinclude a temperature compensation structure electrically andmechanically coupled to the body. The temperature compensation structuremay include a first beam suspended from the substrate, the first beambeing formed from a first material having a first Young's modulustemperature coefficient. The temperature compensation structure mayinclude a second beam suspended from the substrate. The second beam maybe formed from a second material having a second Young's modulustemperature coefficient. The temperature compensation structure mayinclude a routing spring suspended from the substrate. The routingspring may be coupled to the first beam and the second beam. The routingspring may be formed from the second material and have a compliancesubstantially greater than a compliance of the first beam or thecompliance of the second beam.

In at least one embodiment of the invention, a method of manufacturingan apparatus includes forming a MEMS device including a body suspendedfrom a substrate. The MEMS device includes a first electrode suspendedfrom the substrate. The first electrode and the body form a firstelectrostatic transducer. The MEMS device includes a second electrodesuspended from the substrate. The second electrode and the body form asecond electrostatic transducer. The first and second electrodes aremechanically coupled to the body. The method of manufacturing mayinclude forming a suspended passive element mechanically coupled to thebody and electrically isolated from the body. Forming the MEMS devicemay further include forming a temperature compensation structureelectrically and mechanically coupled to the body. The temperaturecompensation structure may include a first beam suspended from thesubstrate. The first beam may be formed from a first material having afirst Young's modulus temperature coefficient. The temperaturecompensation structure may include a second beam suspended from thesubstrate. The second beam may be formed from a second material having asecond Young's modulus temperature coefficient. The temperaturecompensation structure may include a routing spring suspended from thesubstrate. The routing spring may be coupled to the first beam and thesecond beam. The routing spring may be formed from the second materialand have a compliance substantially greater than a compliance of thefirst beam or the compliance of the second beam.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a conventional MEMS device including an in-planeresonator.

FIG. 2 illustrates a circuit diagram of a MEMS device configured as anoscillator.

FIG. 3 illustrates an exemplary cross-sectional view of a MEMS structureprior to a release of a structural layer to form suspended portionsconsistent with at least one embodiment of the invention.

FIG. 4A illustrates a diagram modeling a typical MEMS transducer.

FIG. 4B illustrates a diagram modeling a MEMS transducer having asuspended electrode and a suspended resonator consistent with at leastone embodiment of the invention.

FIG. 4C illustrates a diagram modeling a MEMS transducer having asuspended electrode and a suspended resonator in a coupled electrodeconfiguration consistent with at least one embodiment of the invention.

FIG. 5 illustrates a plan view of a MEMS transducer having a suspendedelectrode and resonator in a coupled electrode configuration consistentwith at least one embodiment of the invention.

FIGS. 6A-6D illustrates various features of the MEMS transducer of FIG.5 having a suspended electrode and resonator in a coupled electrodeconfiguration consistent with at least one embodiment of the invention.

FIG. 6E illustrates static deflection after stress relief of the MEMStransducer of FIG. 5 having a suspended electrode and resonator in acoupled electrode configuration consistent with at least one embodimentof the invention.

FIG. 7A illustrates dynamic mode shape of a high frequency MEMStransducer without suspended electrodes.

FIG. 7B illustrates a plan view of the high frequency MEMS transducerwithout suspended electrodes of FIG. 7A.

FIGS. 8A and 8B illustrate a plan view of a high frequency MEMStransducer with suspended electrodes consistent with at least oneembodiment of the invention.

FIG. 9 illustrates a coupled electrode device consistent with at leastone embodiment of the invention.

FIGS. 10A and 10B illustrate a suspended resistor device consistent withat least one embodiment of the invention.

FIG. 10C illustrates a suspended resistor device and a MEMS transducerhaving lateral electrodes consistent with at least one embodiment of theinvention.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

Referring back to FIG. 1, MEMS device 100 may be modeled as aspring-mass system having a resonant frequency,

${f_{0} = {\frac{1}{2\pi}\sqrt{\frac{k}{m}}}},$

where k is a constant indicative of the spring stiffness, m is mass ofthe resonator, and f₀ is the resonant frequency. In general, the qualityfactor, Q, characterizes a resonator's bandwidth relative to its centerfrequency. The quality factor may be represented as Q=2πf_(o)m/γ, whereγ is damping coefficient (e.g., due to fluid in a cavity surrounding themass). A higher Q indicates a lower rate of energy loss relative to thestored energy of the resonator, i.e., oscillations die out more slowly.An oscillator with a higher Q resonates with higher amplitude but for asmaller range of frequencies around that frequency over smallerbandwidth. To achieve a high-precision, low-power resonator, a high massmay be desired so that the device can have a high stiffness. Increasingmass m increases the quality factor of the resonator if the otherrelevant parameters for Q are held constant. To maintain a particularresonant frequency, an increase in m requires a corresponding increasein k. Other design goals for particular MEMS applications may includelow-frequency operation (e.g., f₀<1 MHz) and insensitivity to shocks tothe housing of the MEMS oscillator. A high stiffness reduces sensitivityto resonator voltage and reduces nonlinearities in operation. However,device characteristics influencing transducer linearity and mechanicalstiffness can be altered by environmental changes, which may affect theinitial accuracy of frequency and stability of frequency in response toaging and temperature variation.

The typical MEMS device of FIG. 1 is made of a movable free standingbody, and one or more electrodes, all of which may be at differentelectrical potentials. A combination of an electrode and a body forms anelectro-mechanical transducer. The electrostatic transducer formed by anelectrode and mass is subjected to environmental factors liketemperature variation and mechanical strain which can influence itsproperties and in turn affects critical performance of the overall MEMSdevice, like initial accuracy. The typical MEMS device uses two separatebodies for an electrode and a movable mass. In general, these bodies areanchored separately from each other and as a result, the transducer gapformed between the two (having a width d) is susceptible to strainresulting from residual stress from the structural layer, coefficient ofthermal expansion mismatch between a structural layer (e.g., SiGe) andthe substrate (e.g., Si), and/or stress from the package, which willgenerally vary with temperature, and possibly with time (e.g., stressrelaxation).

Such variation in transducer properties impacts electromechanicalbehavior and may manifest itself in frequency variation due to strain inresonator applications or acceleration offset and sensitivity ininertial sensor applications and may limit performance or tolerances ofa MEMS system. For example, in a typical MEMS resonator application, aMEMS oscillator may be calibrated to meet a +/−10, 20, or 50parts-per-million (ppm) at room temperature and over temperature. If thetransducer is sensitive to strain, package strain variations due tosolder reflow or temperature variations will modify the electrostaticforce of the transducer, which affects the resonant frequency of thedevice. Frequency variation due to strain variation may result in theMEMS resonator failing to meet a target specification.

Ideally, to reduce the effects of strain on a MEMS device, electrodeanchors and resonator anchors can be placed as close to each other aspractical. As referred to herein, an anchor is a structure thatmechanically couples (e.g., anchors, fixes, fastens, joins, connects, orattaches) a portion of a first structure to a portion of a secondstructure. Portions of the first and second structures that aremechanically coupled have restricted motion. In conventional MEMSdevices, locating the electrode anchors and resonator anchors in thesame location on a substrate is not typically feasible. Therefore, theelectrode and resonator anchors of the MEMS device are located in closeproximity (e.g., as close as allowable by design constraints (e.g.,design rules) for the target manufacturing process) to reducesensitivity of MEMS device to the effects of strain on the MEMS device.Transduction of the MEMS device is often based on the voltage differenceacross the transduction gap (i.e., the voltage difference between themass and the electrode, V_(ME)). For example, the transduction gap of anexemplary MEMS device is defined by the distances between the capacitivefingers of the resonator and corresponding capacitive fingers of anelectrode, which may be equal. Those distances may vary as a function ofstrain, causing a change to the capacitive transduction of MEMS deviceand thus causing a change to the resonant frequency.

A technique decouples transducers of the MEMS device from sources ofstrain by forming a MEMS structure with suspended electrodes that aremechanically anchored in a manner that reduces or eliminates thetransfer of strain from the substrate into the structure (e.g., by usingone or more center anchor structure), or transfers strain to bothelectrodes and body in such a way that the transducer isstrain-tolerant. The technique includes using an electrical insulatormaterial (e.g., SiO₂) embedded in a conductive structural material(e.g., SiGe) both for mechanical coupling and electrical isolation. Asreferred to herein, a structural layer is a layer of a particularmaterial that is later patterned and at least partially released to format least a portion that is free to mechanically move or be deflected inat least one direction with respect to a surface of a substrate. Asreferred to herein, a release of a structure or a portion of astructural layer frees that structure or portion of the structural layerto have a portion that is free to mechanically move or be deflected inat least one directional plane with respect to the substrate. A releaselayer is a layer of material that, when removed, releases at least aportion of the structure or structural layer. The release typicallyoccurs towards the end of manufacture to maintain integrity of thereleased structures.

The embedded electrical insulator material may also be used fortemperature compensation of MEMS devices, which is described in U.S.Pat. No. 7,639,104, filed Mar. 9, 2007 (issued Dec. 29, 2009), entitled“Method for Temperature Compensation in MEMS Resonators with IsolatedRegions of Distinct Material,” naming Emmanuel P. Quevy et al., asinventors, which application is incorporated herein by reference. Theembedded electrical insulator material may be used to electricallyisolate specific areas of the structural layer. The embedded electricalinsulator may be used to route different signals through the structurallayer while keeping a continuous (i.e., monolithic) mechanical body.Although the technique is described using silicon dioxide, otherelectrically insulating materials may be used. Techniques for formingthe electrically insulating material structures (e.g., embedded silicondioxide slits) are described in U.S. Pat. No. 7,639,104 and described inU.S. Pat. No. 7,514,760, entitled “IC-Compatible MEMS Structure” namingEmmanuel P. Quevy as inventor, filed Mar. 9, 2007, issued Apr. 7, 2009,which application is incorporated herein by reference.

One or more embedded electrical insulator slits may be used to routesignals and perform electrical, thermal, and mechanical functionssimultaneously. In at least one embodiment of a MEMS device, embeddedinsulator material is used to form a monolithic MEMS device, whichincludes a self-referenced transducer gap, i.e., the electrode andmovable body are mechanically coupled to move together, thereby reducingthe impact of environmental strain. In at least one embodiment of a MEMSdevice formed using embedded insulator material, an electrode is part ofthe movable body and contributes to the mode shape. In at least oneembodiment of a MEMS device formed using embedded insulator material,the electrode is a movable body and contributes to the relativedisplacement of body versus electrode. As a result, those MEMS devicesmay have more compact designs with higher performance (e.g., highersignal-to-noise ratio versus area). In addition, the embedded electricalinsulator material slit technique allows the routing of separate signalswithin the same structural layer.

Referring to FIG. 3, an exemplary MEMS device that achieves high-Qoperation is manufactured using techniques that form body and electrodestructures that are suspended from a substrate. Manufacturing techniquesthat may be used to produce MEMS devices are described in U.S. Pat. No.7,514,760, filed Mar. 9, 2007, entitled “IC-Compatible MEMS Structure,”naming Emmanuel P. Quevy as inventor; U.S. patent application Ser. No.13/075,800, filed Mar. 30, 2011, entitled “Technique for Forming a MEMSDevice,” naming Emmanuel P. Quevy et al., as inventors; and U.S. patentapplication Ser. No. 13/075,806, filed Mar. 30, 2011, entitled“Technique for Forming a MEMS Device Using Island Structures,” namingEmmanuel P. Quevy et al., as inventors, which applications areincorporated herein by reference. For example, structural layer 302includes structural portions 304 and 306 that are electrically isolated,but mechanically coupled to each other using embedded isolation oxide308. Upon release of the structural material, structural portions 304and 306 are suspended from substrate 312. Structural portions areelectrically coupled to electrical domains using electrical contactstructures 314, 316, and 318. Signals may be routed using embeddedinsulator portions, while maintaining a continuous (i.e., monolithic)mechanical body as discussed further below.

Referring to FIG. 4A, a typical electrostatic transducer includes amovable mass 402 anchored to a substrate 420 and a movable electrodeanchored to the substrate 420. The electrostatic transducer has atransducer gap, d, a resonator-electrode voltage V_(re), aresonator-electrode capacitance C_(re), and has an efficiency of

${{{\Gamma = {V_{re}\frac{\partial C_{re}}{\partial x}\frac{\partial x}{\partial t}}},{{{where}\mspace{14mu} C_{re}} = {\frac{ɛ_{0}A}{d_{0} - x}\mspace{14mu} {and}\mspace{14mu} \frac{\partial C_{re}}{\partial x}}}}}_{x = 0} = {\frac{C_{re}(0)}{d_{0}}.}$

Frequency variation due to strain, etc., results in a resonant frequencyof f_(osc)=f_(mech)*√{square root over (1−β(d)V²)}, where straindependence (from residual stress and thermal stress) of frequencyaccuracy is

${{\delta ɛ}( {\sigma,T} )} = {\frac{ɛ}{d_{0}}.}$

The resulting frequency shift due to strain is:

$\frac{\Delta \; f}{f}{{ ( {\delta \; ɛ} ) \sim\frac{1}{2}} \cdot {\frac{3\delta \; {ɛ \cdot {\beta ( d_{0} )} \cdot V^{2}}}{1 - {{\beta ( d_{0} )} \cdot V^{2}}}.}}$

Frequency control applications may attempt to reduce or eliminate thisfrequency shift by reducing or eliminating the frequency variation dueto strain and/or reducing or eliminating the strain communicated to thedevice transducer.

Referring to FIG. 4B, a suspended electrode (represented by plate 422coupled to a spring-damper system) and suspended body (represented bymass 424 coupled to a spring-damper system) configuration results in atransducer gap, d, that only depends on the topological mismatch betweenelectrode and body. The suspended electrode and body are electricallyisolated and mechanically coupled by electrically insulating material406 in mechanical suspension 408. The topological mismatch (e.g.,electrode and body mismatch due to stress) can be tailored or nulled outby design. Referring to FIG. 4C, in a coupled electrode resonant mode,each electrode is a resonant body and is mechanically coupled to theother electrode to achieve the target resonant mode shape. Theelectrodes are electrically isolated and mechanically coupled usingelectrically insulating material 410 in mechanical suspension 412.

FIG. 5 illustrates an exemplary embodiment of a low frequency resonatorhaving a suspended electrode and a suspended body (i.e., resonator ormass). Low frequency resonators are typically sensitive to frequencyshift due to strain because of their low stiffness. MEMS device 500includes a comb drive transducer, which is used in some applications forimproved linearity of capacitance as a function of displacement. Theelectrodes and resonator of MEMS device 500 include electricallyconductive comb structures, which include rows of electrode teeth thatinterlock, but do not touch, rows of body teeth. The body and electrodesmove longitudinally, in-plane. However, techniques described herein maybe adapted to embodiments of a MEMS device including parallel platetransducers and/or those in which the body and/or electrodes moveout-of-plane.

Still referring to FIG. 5, in MEMS device 500, suspension beams 512 and513 include signal routing that extends through center anchor 504 ofbody 502. Body 502 includes folded beam springs. Alternately, thesuspension beams made from different materials may be configured inparallel physically and mechanically, instead of the folded spring shownhaving suspension beams mechanically in series but physically parallel.In the alternate case, the signal routing portion is not needed.Location of center anchor 504 in the center of the body reduces transferof mechanical stress to the vibrating structure. Center anchor 504 is amonolithic structure, including multiple mechanically coupled portions.However, center anchor 504 is partitioned into multiple electricallyisolated portions (e.g., portions including contacts 503, 505, 507, 509,and 511 to separate electrical domains for a first electrode, a body,and a second electrode, described further below with reference to FIGS.6A-6C). Referring back to FIG. 5, the relatively large mass per area ofthe body of MEMS device 500 increases the stiffness of the folded beamfor a given frequency, thereby reducing frequency variation due tostrain as compared to MEMS devices having less mass for the same targetfrequency. Body 502 of MEMS device 500 also includes embedded embeddedelectrical insulator material slits 522 to match the static deflectionof the suspension beam in order to align individual transducer facesonce residual stress is relieved in the entire structure, as discussedfurther below. Note that in FIG. 5, electrically insulating material isshaded with hatching and electrically conductive material is shaded withdots. The unshaded gaps, e.g., between the electrode teeth and bodyteeth, may contain air or other fluid.

Referring to FIGS. 5 and 6A-6E, various features of MEMS device 500 areillustrated. Referring to FIG. 6A, electrically conductive portions ofbody 502 are coupled to a first electrical domain via contact 503 ofcenter anchor 504. Body 502 is suspended from the substrate bysuspension beams 512 and 513 extending between center anchor 504 andends of body 502. The hatched regions of MEMS device 500 in FIG. 6Aindicate those portions of MEMS device 500 that are coupled to the firstelectrical domain, other portions of MEMS device 500 (electricallyconductive and electrically insulating) are indicated with dots, andunshaded gaps, e.g., between the electrode teeth and body teeth, maycontain air or other fluid. Referring back to FIG. 5, conductiveportions of suspension beams 512 and 513 are delineated as part of thefirst electrical domain by electrically insulating material embedded insuspension beams 512 and 513 (indicated by hatching in FIG. 5). Otherportions of suspension beams 512 and 513 are electrically isolated fromthe first electrical domain by that embedded electrically insulatingmaterial. Body 502 includes conductive finger structures that areinterdigitated with conductive finger structures of electrodes to formcomb drive transducers.

Referring to FIG. 6B, a first electrode of MEMS device 500 is coupled toa second electrical domain via contacts 505 and 507 of center anchor504. Portions of the first electrode are suspended from the substrate bysuspension beam 512 extending between the center anchor structure andends of the body. The hatched regions of MEMS device 500 in FIG. 6Bindicate those portions of MEMS device 500 that are coupled to thesecond electrical domain, other portions of MEMS device 500(electrically conductive and electrically insulating) are indicated withdots, and unshaded gaps, e.g., between the electrode teeth and bodyteeth, may contain air or other fluid. Referring back to FIG. 5,suspension beam 512 includes conductive portions that are defined aspart of the second electrical domain by electrical isolation material515 embedded in suspension beam 512. However, other portions ofsuspension beam 512 are electrically isolated from the second electricaldomain by that embedded electrical isolation material. The firstelectrode includes conductive finger structures that are interdigitatedwith conductive finger structures of body 502 to form comb drivetransducers.

Referring to FIG. 6C, a second electrode of MEMS device 500 is coupledto a third electrical domain via contacts 509 and 511 of center anchor504. Portions of the second electrode are suspended from the substrateby suspension beam 513 extending between the center anchor structure andends of body 502. The hatched regions of MEMS device 500 in FIG. 6Cindicate those portions of MEMS device 500 that are coupled to the thirdelectrical domain, other portions of MEMS device 500 (electricallyconductive and electrically insulating) are indicated with dots, andunshaded gaps, e.g., between the electrode teeth and body teeth, maycontain air or other fluid. Referring back to FIG. 5, suspension beam513 includes conductive portions that are defined as part of a thirdelectrical domain by electrical isolation material 515 embedded insuspension beam 513. However, other portions of suspension beam 513 areelectrically isolated from the third electrical domain by embeddedelectrical isolation material. The second electrode includes conductivefinger structures that are interdigitated with conductive fingerstructures of body 502 to form the comb drive transducers.

Still referring to FIG. 5, MEMS device 500 includes a folded beamstructure. As discussed above, MEMS device 500 includes electricallyinsulating material portions (indicated by hatching in FIG. 5) in thesuspension beams 512 and 513 to allow separate signal routing to thebody and the electrodes. Those electrically insulating material portionsmay affect the strain gradient of the suspension beams. If the straingradients between the suspension beams and the body are not matched,those electrically insulating material portions may cause the suspensionbeams to curl differently with respect to the body, resulting in amisalignment of the electrode and body at the transducer gap of the combdrive transducers, thereby reducing transducer efficiency. Referring toFIGS. 5 and 6E, to compensate for changes in strain gradient withrespect to the body due to electrical insulator material in suspensionarms of MEMS device 500, body 502 includes embedded electrical insulatormaterial slits 522. Those electrically insulating material slits havegeometries that match a strain gradient of the body to the straingradient in the suspension beams. As a result of the design features ofMEMS device 500, the static deflection of portions of MEMS device 500including the electrodes is matched to the static deflection of the bodyin the out-of-plane direction, as illustrated by the static deflectionmap of FIG. 6E.

Referring to FIGS. 7A and 7B, MEMS device 700 is an exemplaryhigh-frequency resonator including a suspended mass having multipleanchors but without suspended electrodes. The displacement profile ofFIG. 7A illustrates the target mode shape of the basic shape structure.Each of electrodes 712, 714, 716, and 718 are independently anchored tothe substrate by multiple anchors, which include contacts tocorresponding electrical domains. Plate resonator 702 is separatelyanchored to the substrate by a five-point anchoring technique includingcentral anchor 720 and an anchor 722 at each corner of the plate. Eachcorner anchor includes a decoupling spring 724 and an electricalconnection to a corresponding domain.

FIGS. 8A and 8B illustrate an exemplary high-frequency MEMS device 800including a suspended body and suspended electrodes that aremechanically coupled to each other using electrically insulatingmaterial portions 806. Plate resonator 802 is anchored to the substrateby a central anchor, which also includes an electrical contact to afirst electrical domain. The four corner anchors are mechanicallycoupled to plate resonator 802 using electrically insulating materialportions, but electrically isolated from plate resonator 802 byelectrically insulating material portions. Each of the corner anchorselectrically couples an electrical domain to one of the electrodes(e.g., electrode 816) mechanically coupled to the anchor. Each corneranchor also includes an electrically insulating material portion 808that electrically isolates that electrical domain from another electrodemechanically coupled to the anchor (e.g., electrode 818). Thus, thedesign of MEMS device 800 reduces or eliminates frequency variation dueto strain.

The techniques described above may be applied to other types of MEMSdevices. Electrodes may be suspended above a substrate and mechanicallyreferenced with respect to a suspended resonator beam to reducefrequency variation due to strain. Referring to FIG. 9, a flexuraldevice includes suspended body beam 904 between suspended electrode beam902 and suspended electrode beam 906. Unlike MEMS devices 500, 700, and800, which may be modeled as the system of FIG. 4B having stationaryelectrodes, MEMS device 900 includes a body and electrodes that vibratetogether (i.e., form a vibration mode together). In MEMS device 900,suspended electrodes beams 902 and 906 are tuning-fork-like structuresthat in conjunction with suspended body beam 904, form a resonator and atransducer at the same time. Such beam structures may be used ingyroscopes and low-frequency timing structures. Suspended body beam 904and suspended electrode beams 902 and 906 are electrically isolated andmechanically coupled by electrically insulating material portion 908.Any substrate strain causes the electrodes to move together. Thus theelectrodes are self-referenced and MEMS device 900 is straininsensitive. Each of the beams is associated with a different electricaldomain, but are mechanically coupled to each other. Electricallyinsulating material portion 908 mechanically couples suspended body beam904 and the suspended electrode beams 902 and 906. Routing signal 918travels through electrically insulating material portion 908 and couplessuspended body beam 904 to anchors 910 and 912, which are mechanicallydecoupled from suspended body beam 904 by decoupling springs 914 and 916to reduce transfer of strain from the substrate to suspended body beam904 and include electrical contacts to a corresponding electricaldomain. Unlike conventional MEMS structures, the electrode beams andbody beam of MEMS device 900 vibrate together and device 900 has reducedstrain sensitivity as compared to conventional MEMS devices since anymovement due to strain will cause the electrode beams and body beam tomove in a similar manner. Although the mechanical coupling andelectrical isolation of electrodes and body structures are illustratedfor a flexural device, the techniques described herein may be adaptedfor bulk acoustic mode devices in which the electrically insulatingmaterial structure may be designed as part of the mode shape.

Referring back to FIGS. 5 and 6D, in at least one embodiment, MEMSdevice 500 includes multiple temperature compensation structures,portions of which are hatched in FIG. 6D. An individual temperaturecompensation structure includes independent beams having differentstiffness variations as a function of temperature (i.e., differentYoung's modulus temperature coefficients). In at least one embodiment ofthe temperature compensation structure, a first material forming beam508 has a different Young's modulus temperature coefficient than secondmaterial forming beam 506. The Young's modulus temperature coefficientof the first material need only be different than that of the secondmaterial over the operational range of the MEMS device. Any materialhaving a different Young's modulus temperature coefficient than firstmaterial over the typical operating range (e.g., approximately −40° C.to approximately 85° C. or approximately −55° C. to approximately 125°C.) may be employed as second material. In at least one embodiment ofthe temperature compensation structure, the second material has anegative Young's modulus temperature coefficient, while the firstmaterial has a positive Young's modulus temperature coefficient. In atleast one embodiment of the temperature compensation structure, beamspring 508 is formed from the structural material, which may be asemiconductor such as, but not limited to, silicon (Si), germanium (Ge),and SiGe alloys, and beam spring 506 is formed from the electricallyinsulating material, which may be SiO₂ and indicated by hatching inFIGS. 5 and 6D. Note that silicon dioxide has the unusual property ofbecoming stiffer as temperature increases. In other embodiments of thetemperature compensation structure, beam spring 506 is formed from othermaterials, which may have positive or negative Young's modulustemperature coefficients.

The temperature compensation structure may include routing spring 510,which dominates the electrical behavior of the temperature compensationstructure. That is, routing spring 510 is a serpentine structure formedfrom the structural material and serves to electrically couple the bodyto the electrical domain of the center anchor. Routing spring 510 has amuch higher compliance than beam spring 506 and beam spring 510. Thusrouting spring 510 does not substantially influence the mechanicalbehavior but dominates the electrical behavior of the temperaturecompensation structure. Beam spring 506 and beam spring 508 have ahigher stiffness than routing spring 510 and thus dominate themechanical behavior of the temperature compensation structure. Note thatthe beam springs may be coupled mechanically in series or in parallel toform a spring that supports the movable body and beam springs androuting spring may have other geometries. The temperature compensationstructure is selectively located to specific regions of MEMS device 500and beam springs 506 and 508 are dimensioned to modify the temperatureresponse of MEMS device 500 (e.g., the temperature coefficient offrequency of a resonator) independent of other properties of the MEMSdevice (e.g., a resonator mode shape).

This approach can simplify design as compared to other temperaturecompensation techniques that use strips of silicon dioxide surrounded bysilicon germanium on either side or surrounded by strips of silicongermanium. The dual beam technique may also substantially reduce theamount of interface between the two materials. The silicongermanium-silicon dioxide interface can introduce undesirable effectssuch as locally varying properties and the generation of mechanical weakpoints and stress concentrations. Having separate spring portionsfacilitates moving structural weak points and stress concentrationfeatures to less critical locations. The separate beam approach totemperature compensation can reduce thermoelastic energy losses and thusdamping at silicon germanium-silicon dioxide interface, therebyincreasing the quality factor of the resonator, which is a metric forshort-term stability. The separate beam technique may also improvedesign flexibility by allowing independent selection of silicon dioxideand silicon germanium beam dimensions, thereby expanding the designspace available to achieve temperature compensation at any particularfrequency. Unlike temperature compensation techniques that use strips ofoxide surrounded by other material, the dual-beam technique may beeasily adapted to compensate for effects of temperature variations inMEMS devices that use slender flexural beams, e.g., inertial sensors. Inaddition, the dual-beam technique is less sensitive to somemanufacturing tolerances, e.g., pattern alignment of silicon dioxide tosilicon germanium.

Referring to FIGS. 10A and 10B, electrical insulator material embeddedin a MEMS structural layer is used to form suspended resistor 1002 thatis mechanically coupled to a central beam of the resonator. Embeddedelectrical insulator traces 1006 and 1007 electrically isolate andmechanically couple a serpentine portion of structural material tracesto form suspended resistors 1002 and 1003. Suspended resistors 1002 and1003 are mechanically anchored to the substrate by contacts 1008 and1010 and contacts 1012 and 1014, respectively. Contacts 1008 and 1010and contacts 1012 and 1014 serve as the electrical terminals ofsuspended resistors 1002 and 1003, respectively.

Suspended resistors 1002 and/or 1003 may be configured to maintainconstant or otherwise adjust a temperature of MEMS device 1000 byregulating power dissipation into the body 1020. Suspension of theresistor from the substrate improves thermal isolation and using arelatively small thermal mass as compared to heaters that are embeddedin a substrate. The smaller thermal mass is needed to make wafer levelcalibration practical by keeping heating currents low. The suspendedresistor allows on-chip, wafer-level calibration of a resonator with arelatively small thermal mass.

In another embodiment of MEMS device 1000, suspended resistors 1002and/or 1003 are thermally coupled to MEMS device 1000 and configured tocharacterize the temperature response of MEMS device 1000. For example,suspended resistors 1002 and 1003 may be used as a temperature sensorelement (i.e., a thermistor) of a bridge temperature sensor. A change intemperature of body 1020 will cause a corresponding change in theresistance of suspended resistors 1002 and 1003 by an amountcharacterized by the temperature coefficient of resistance (TCR) of thestructural material (e.g., SiGe). That temperature change can bedetermined based on a voltage drop across the resistor, a predeterminedresistance value at a predetermined temperature, and the TCR of thestructural material. When used as a temperature sensor, the suspendedresistor technique allows placement of the sensing element proximate tothe element (e.g., MEMS resonator 1000) that is having its temperaturesensed and/or compensated. Mechanically coupling the resistor to thecentral beam of the device, via electrically isolating materialportions, reduces effects of strain on the temperature measurement. Suchplacement reduces thermal gradients and associated temperaturemeasurement errors, which results in a sensor that is more accurate overtemperature than other sensors and measures temperature where itmatters, i.e., at the location of the device that needs compensation fortemperature shifts.

MEMS device 1000 is an exemplary lattice transducer that may beconfigured to generate a target frequency using a torsional mode or aflexural mode. MEMS device 1000 includes body 1020 formed from latticebeams that are suspended from the substrate. Body 1020 is anchored tothe substrate by anchors 1016 and 1018, which are coupled to body 1020by decoupling springs. Anchors 1016 and 1018 also provide electricalcontact to the body 1020. The lattice beams of body 1020 formsubstantially square lattice openings that surround, but do not touch,corresponding electrodes that are anchored to the substrate. Eachsubstantially square lattice opening forms a transducer gap along theperimeter of the opening and the perimeter of the substantially squareelectrode 1022. Note that the perimeter of the lattice openings and theperimeter of the electrodes have a varied perimeter that increases thearea of the transducers. However, the perimeters of the lattice openingsand the electrodes may have other geometries. MEMS device 1000, asillustrated, includes eighteen transducers on each side of the centeranchors 1016 and 1018, however, the number of transducers is exemplaryonly and may vary according to application.

Although the suspended passive element technique is illustrated in anembodiment including a lattice transducer, the suspended passive elementtechnique may be incorporated into MEMS devices having any type oftransducers and/or having mechanically coupled electrodes and body(e.g., MEMS device 500). For example, FIG. 10C illustrates the suspendedpassive resistor in an embodiment including beams 1038 and 1040 andlateral electrodes 1030, 1032, 1034, and 1036. In addition, althoughtechniques described above illustrate the use of insulator materialembedded in a MEMS structural layer to form electrostatic transducersand suspended resistors, the technique may be used for other types ofpassive elements, which may be used in filters, switches, or otherapplications. For example, by using a structural material that has a lowresistivity, the technique may be used to form suspended inductors(e.g., planar spiral inductors) that have a high quality factor (i.e.,low eddy currents) or to form electromechanical switches that are strainfree for improved manufacturability, lower switching voltages, andimproved reliability (e.g., reduced risk of stiction).

The description of the invention set forth herein is illustrative, andis not intended to limit the scope of the invention as set forth in thefollowing claims. For example, while the invention has been described inembodiments in which specific MEMS structures (e.g., comb drive,parallel plate, and lattice transducers) and materials (e.g., SiGe andSiO₂) are used, one of skill in the art will appreciate that theteachings herein can be utilized with other types of MEMS structures andmaterials. Variations and modifications of the embodiments disclosedherein, may be made based on the description set forth herein, withoutdeparting from the scope and spirit of the invention as set forth in thefollowing claims.

What is claimed is:
 1. An apparatus comprising: a microelectromechanicalsystem (MEMS) device comprising: a body suspended from a substrate; afirst electrode suspended from the substrate, the first electrode andthe body forming a first electrostatic transducer; and a secondelectrode suspended from the substrate, the second electrode and thebody forming a second electrostatic transducer, the first and secondelectrodes being mechanically coupled to the body.
 2. The apparatus, asrecited in claim 1, wherein the body, the first electrode, and thesecond electrode are coplanar structures formed in a structural layerformed using the substrate.
 3. The apparatus, as recited in claim 1,wherein the MEMS device further comprises: an anchor mechanicallycoupling the first electrode, the second electrode, and the body to thesubstrate.
 4. The apparatus, as recited in claim 3, further comprising:a suspension portion electrically and mechanically coupling the body tothe anchor.
 5. The apparatus, as recited in claim 4, wherein thesuspension portion comprises a folded spring including suspension beamscoupled mechanically in series and physically in parallel.
 6. Theapparatus, as recited in claim 3, wherein the anchor comprises: first,second, and third anchor portions mechanically coupled to each other andelectrically isolated from each other.
 7. The apparatus, as recited inclaim 3, wherein the anchor comprises: electrical insulator portionsdefining portions of the first electrode, the second electrode, and thebody; and electrical conductor portions forming portions of the firstelectrode, the second electrode, and the body.
 8. The apparatus, asrecited in claim 3 wherein the anchor is disposed substantially at acenter of the body.
 9. The apparatus, as recited in claim 1, wherein theMEMS device further comprises: a plurality of anchors mechanicallycoupling the body to the substrate, each of the plurality of anchorscomprising a decoupling spring, mechanically decoupling the body from acorresponding anchor.
 10. The apparatus, as recited in claim 9, whereinthe MEMS device further comprises: a coupling structure mechanicallycoupling at least one of the first and second electrodes to the body.11. The apparatus, as recited in claim 1, further comprising: asuspension portion mechanically coupled to the anchor and the body,wherein the body comprises a first material and slits of a secondmaterial embedded in the first material, the slits of the secondmaterial compensating for static deflection of the suspension portion.12. The apparatus, as recited in claim 1, wherein the body comprises aframe surrounding the first and second electrodes.
 13. The apparatus, asrecited in claim 1, further comprising: a suspended passive elementmechanically coupled to the body and electrically isolated from thebody.
 14. The apparatus, as recited in claim 13, wherein the suspendedpassive element is a suspended resistor comprising a serpentinestructural material portion attached to electrically insulating materialportions.
 15. The apparatus, as recited in claim 14, further comprising:a bridge temperature sensor circuit comprising the suspended resistorand configured to detect a temperature change of the MEMS device. 16.The apparatus, as recited in claim 14, further comprising: a circuitcomprising the suspended resistor, the circuit being configured toadjust a temperature of the MEMS device.
 17. The apparatus, as recitedin claim 1, further comprising: a temperature compensation structureelectrically and mechanically coupled to the body, wherein thetemperature compensation structure comprises: a first beam suspendedfrom the substrate, the first beam being formed from a first materialhaving a first Young's modulus temperature coefficient; a second beamsuspended from the substrate, the second beam being formed from a secondmaterial having a second Young's modulus temperature coefficient; arouting spring suspended from the substrate, the routing spring beingcoupled to the first beam and the second beam, the routing spring beingformed from the second material and having a compliance substantiallygreater than a compliance of the first beam or the compliance of thesecond beam.
 18. A method of manufacturing an apparatus comprising:forming a microelectromechanical system (MEMS) device comprising: a bodysuspended from a substrate; a first electrode suspended from thesubstrate, the first electrode and the body forming a firstelectrostatic transducer; and a second electrode suspended from thesubstrate, the second electrode and the body forming a secondelectrostatic transducer, the first and second electrodes beingmechanically coupled to the body.
 19. The method of manufacturing anapparatus, as recited in claim 18, further comprising: forming asuspended passive element mechanically coupled to the body andelectrically isolated from the body.
 20. The method of manufacturing anapparatus, as recited in claim 18, wherein forming the MEMS devicefurther comprises: forming an anchor mechanically coupling the firstelectrode, the second electrode, and the body to the substrate.
 21. Themethod of manufacturing an apparatus, as recited in claim 20, whereinforming the anchor comprises: forming electrical insulator portionsdefining portions of the first electrode, the second electrode, and thebody; and forming electrical conductor portions forming portions of thefirst electrode, the second electrode, and the body.
 22. The method ofmanufacturing an apparatus, as recited in claim 18, wherein forming theMEMS device further comprises: forming a temperature compensationstructure electrically and mechanically coupled to the body, wherein thetemperature compensation structure comprises: a first beam suspendedfrom the substrate, the first beam being formed from a first materialhaving a first Young's modulus temperature coefficient; a second beamsuspended from the substrate, the second beam being formed from a secondmaterial having a second Young's modulus temperature coefficient; arouting spring suspended from the substrate, the routing spring beingcoupled to the first beam and the second beam, the routing spring beingformed from the second material and having a compliance substantiallygreater than a compliance of the first beam or the compliance of thesecond beam.
 23. An apparatus comprising: means for vibrating, whereinthe means for vibrating is suspended from a substrate; means forelectrostatically driving the means for vibrating; means forelectrostatically sensing vibrations of the means for vibrating, whereinat least one of the means for electrostatically driving and the meansfor electrostatically sensing is suspended from the substrate; and meansfor mechanically coupling and electrically isolating the means forvibrating and at least one of the means for electrostatically drivingand the means for electrostatically sensing.
 24. The apparatus, asrecited in claim 23, further comprising: means for anchoring to thesubstrate the means for vibrating and the at least one of the means forelectrostatically driving and the means for electrostatically sensing;and means for suspending the means for vibrating from the means foranchoring.
 25. The apparatus, as recited in claim 23, furthercomprising: means for heating the means for vibrating, wherein the meansfor heating is suspended from the substrate; and means for mechanicallycoupling and electrically isolating the means for vibrating to the meansfor heating.
 26. The apparatus, as recited in claim 23, furthercomprising: means for sensing a temperature of the means for vibrating,wherein the means for sensing a temperature is suspended from thesubstrate; and means for mechanically coupling and electricallyisolating the means for vibrating to the means for sensing atemperature.
 27. The apparatus, as recited in claim 23, furthercomprising: means for compensating for variations in frequency ofvibrations due to a change in temperature using a first suspendedstructure being formed from a first material having a first Young'smodulus temperature coefficient and a second suspended structure formedfrom a second material having a second Young's modulus temperaturecoefficient.
 28. The apparatus, as recited in claim 23, furthercomprising: means for compensating for variations in frequency ofvibrations due to strain, wherein the means for compensating is embeddedin the means for vibrating.